Fluid heater



Dec. 8, 1964 MILLER 3,160,145

FLUID HEATER Filed July 20, 1962 13 INVENTOR. $4 41/74. M4452 W States Patent ()fifice 3,16%,145 Patented Dec. 8, 1%64 3,169,145 FLUTE) HEATER Avy L. Miller, 1324-6 Saticoy St., North Hollywood, Calif. Filed duly 2d, 1962, Ser. No. 214,156 14 Eta-inns. (El. 122-367) The present invention relates generally to fluid heaters and heat exchangers, and more particularly to water heaters of the horizontal tube, continuous flow type.

This application is a continuation-in-part of my application Serial No. 144,901 filed October 13, 1961 for: Fluid Heater, now abandoned.

In fiuid heaters and heat exchangers employing heat exchange tubes, it has been proposed to increase the heat transfer area by the use of fins about the exterior surfaces of the tubes. Various baffling arrangements have also been employed with the tubes to secure better contact of the heating gases with the heat transfer surface of the tubes and the fins thereon. Also the tubes have been disposed in superposed relation in banks to provide greater heat transfer area in the direction of flou of the heating gases. All of these arrangements have been subject to various disadvantages. For example, in the use of fins alone, the top or back portions of the fins are substantially ineffective; the bafiles used do not give the desired control of the flow of heating gases; and the use of a bank of tubes lessens the heat transfer by direct radiation to the upper tubes.

The fluid heater according to the present invention utilizes a finned heat exchange tube through which the fluid to be heated flows, but greatly increases the usable area of the fins by directing the flow of the heating or combustion gases to the back or top of the fin to secure a greater combustion gas-fin area contact. The present invention further increases the turbulence in the combustion gas flowing past the heat exchange tube and fins and prevents thermal stratifying of the gas and failure of the hot or remote gases to contact the fins. With the use of the fluid heater according to the present invention, sufficient heat transfer can often be secured with a single row of tubes, so that there is relatively better heat transfer by direct radiation, there being no upper tubes to be shielded by the lower.

In one form of the present invention means are provided for increasing the flow of combustion gases past baffled heat exchange tubes by converting the flow energy of the gases on the downstream side of the heat exchange tubes to pressure energy to increase the flow of combustion gases through the heater flue. This permits an increase in the tube baffling, or a reduction in the size of the flue or the height of the combustion chamber.

It is, therefore, an object of the present invention to provide a fluid heater securing more effective use of the heat transfer area of a heat exchanger.

Another object of this invention is the provision of an improved fluid heater in which stratification of heating gases into layers of higher and lower temperature is prevented and turbulence is imparted to the heating gases to increase uniformity of temperature therethrough and more effective transfer of their heat energy, thus unifonn- 1y cooling the gas and preventing loss of high temperature to the stack.

A further object of this invention is to materially in crease the over-all efiiciency of heat transfer in a fluid heater by providing optimum contact of the heating gases with the heat transfer areas of fluid-flow, heat exchange tubes.

Yet another object of this invention is the provision of an improved fluid heater having a combustion gas collection chamber between heat exchange tubes and the outlet flue in which the combustion gas pressure is increased by the conversion of the flow energy of the gases passing the heat exchange tubes into pressure energy.

These and other objects and features of the invention will be readily apparent to those skilled in the art from the followingspecification and the appended drawing, in which:

FIGURE 1 is a partially cutaway, perspective View of a Water heater embodying the present invention; 1

FIGURE 2 is an enlarged, fragmentary, vertical sectional view of a heat exchange tube arrangement according to the present invention, taken on the line 22 of FIGURE 1;

FIGURE 3 is an enlarged, transverse, vertical sectional View on the line 33 of FIGURE 2;

FIGURE 4 is a view similar to FIGURE 3 showing a modified form of the invention; and

FIGURE 5 is a partial perspective view showing one of the baffle elements of the modification of FIGURE 4.

Referring to FIGURE 1 in the drawing, it will be seen that the fluid heater or heat exchanger embodying the present invention is exemplified by a continuous flow Water heater enclosed in an outer, generally rectangular, sheet metal housing 10, having gas fired burners 11 near the bottom of an internal firebox l2. Horizontal heat exchange tubes 13 extend longitudinally from front to back across the firebox 12 near the top thereof. The burners 11 and the water tubes 13 are completely enclosed by the firebox 12, which may be lined with refractory tile and insulated from the outer housing 10 by a layer of rockwool 14-, or the like. An interior hood 15, of sheet metal, serves to collect the combustion gases after they .have passed in contact with the heat exchange tubes 13 and to direct the cool gases to the flue 16 at the top of the hous ng 16.

Fuel gas is supplied by a fuel line 17; and conventional gas control elements are provided in the form of a manual and automatic pilot control valve 18 connected to the gas supply line 17, a pressure regulator 19, a solenoid control valve Zll for the main burners l1, and a conventional safety pilot 22. Since the just described elements do not form an important part of the present invention, no further detailed description thereof is deemed necessary.

The heat exchange tubes 13 are preferably arranged in a. single row in relatively closely transversely spaced arrangement, as illustrated in the drawing, and are mounted between headers 23 and 24, as by brazing, at 25. The headers 23 and 24 form ends for the firebox 12, and are hollow to provide water manifold chambers 26 and 27 which respectively divide the flow and recombine it as the water passes through the heater. The cold water intake to the heater is at 28 leading to the water manifold chamber 26, and having an internally threaded connection to receive an input pipe, not shown. The outlet from the heater is indicated at 29 leading from the manifold chamber 27, and also internally threaded for the reception of an outlet pipe 31. The heat exchange tubes 13 are formed by tubes or pipes 32 of a relatively good conductor, such as copper, having spiral fins 33 of like material brazed or otherwise intimately mounted on their exterior surfaces in good heat conducting relation, so as to effectively increase the outer heat transfer area of the tubes. The fins 33 may be placed on the tubes 32 as individual annuli mounted thereon, but for ease of manufacture are preferably in the form of continuous spirals or coils brazed to the outer surface of the tubes.

To secure the more eificient heat transfer characteristics according to the present invention, the row of finned heat exchange tubes 13 is provided with T-shaped baflles 35 having top plates 36 whose opposite edges rest upon the tops of the fins 33 of adjacent tubes 13, and which have depending vertical legs 37 disposed between the fins of adjacent tubes. The baflles may be formed by rolling or welding, or may be cast or otherwise constructed of any desired material. Adjacent top plates 36 are. spaced apart, as at 38, to providefor egress of the combustion gases therethrough in their passage from'the burners 11' to the hood 15 and the flue 16. The baffles 35 extend substantially entirely across the firebox 12, between the headers 23 and 24, as shown in FIGURE 2. The depending vertical legs 37 preferably extend downwardly just below the plane of the axes of the tubes 13, and may be relied upon to position the baflles 35 transversely of the tubes by their disposition between the edges of the adjacent fins 33. To secure more positive positioning of the baffles 35 with respect to the heat exchange tubes, slotted mounting lugs 39 may be provided on the opposed walls of the headers 23 and 24, to receive the lower corners of the legs 37.

While the illustrations, in the figures of the drawing show T-shaped baflles 35, it will be understood that the end tubes in the row may be serviced by L-shaped baffles having their vertical legs disposed along the side walls of the firebox and their horizontal legs extending inwardly and resting on the tops of the fins of the adjacent heat exchange tube. Similarly, double L-shaped baffles, with their vertical legs connected, would be the full equivalent of the T-shaped baifles disclosed herein. Also, in the broader aspects of the invention, the vertical leg of the baffle may be omitted.

It will be seen more particularly from an inspection of FIGURE 3, that combustion gases arising through the firebox 12 from the burners 11 will pass over the bottom and side surfaces of the fins 33, as heretofore; but will then be directed by the baffies 35 so that they must pass around the tubes 32 and in contact with the fins 33 adjacent to the tops of the tubes before emerging from the heat transfer zone through the slots 33. The slots 38 are formed by the spaces between the top plates 36 of the baffles 35 at the vertical planes through the axes of the heat exchange tubes and are preferably the minimum Width consistent with a good draft flow through the heater. By having the top plates 36 rest directly upon the top edges of the fins 33, with a minimum spacing between the plates, all of the combustion gases moving past the heat exchange tubes must pass between the fin surfaces at the top of the tubes, thus increasing the usable area of the fins.

Also according to the present invention, the combustion gases passing upwardly through the row of heat exchange tubes 13 are prevented from stratifying in vertical layers wherein portions of the gases in contact with the fins 33 would be cooled to a lower temperature than portions out of contact therewith, permitting escape of the latter portions to the flue at a high and wasteful temperature, In the fluid heater according to the present invention, combustion gases are caught in the sharp corners or pockets between the top plates 36 and the depending legs 37 of the baffles 35 to cause turbulence in the gases in their upward flow, so that higher and lower temperature gases are mixed together. Thus the gases are cooled uniformly and stratification and attendant loss of heat through escape of high temperature gases are avoided. A corollary result is an increase in the temperature differential between the fins and the contacting combustion gases with attendant greater heat transfer. This operation is indicated by the looped turbulence arrows in the corners between the top plates and the depending legs of the bafiles.

A third aspect of the present invention resides in the feature that, with the more efficient and effective utilization of the area of the fins 33, suflicient heat transfer may be effected in the single row of heat exchange tubes 13, so that it is unnecessary to arrange them in multiple layers with the upper tubes shielded from heat transfer by direct radiation from the combustion flame. Therefore, the present invention may be utilized to special advantage in a single row of heat exchange tubes where all receive heat by direct radiation from the combustion flame, and therefore the efficiency of the heat transfer tubes for a given number and area is relatively increased.

By way of example only, and without limitation to the scope of the invention, with heat transfer tubes 13 having an over-all fin diameter of 1% inch, the slot or spacing between the top plates 36 of the baflles 35 may be of the order of inch; while, as noted, the depending central legs 37 preferably extend to a position just below the plane of the axes of the tubes.

In the modification of the present invention shown in FIGURES 4 and 5, the baffle 35 has been replaced by a baflie 51 having a top plate 52 corresponding to the top plate 36 and a vertical depending leg 53 corresponding to the vertical depending leg 37. However, at the edges of the top plate 52 are disposed upwardly extending flanges 54 and 55 which are inclined toward each other out of the vertical plane. When the baffles 51 are mounted above the heat exchange tubes 13, as shown in FIGURE 4, they are spaced apart to provide slots 56 between the edges of adjacent baflies disposed vertically above the axes of the tubes. The baffles 51 thereby provide substantially the same advantages as result from the baffles 35.

The flanges 54 and 55 on adjacent baflles 51 and at opposite sides of the slots 56 form diverging throats 57 between the flanges which act as diffusers to convert the flow energy or velocity of the combustion gases passing through the slots 56 into pressure energy as the gases expand in the diffuser throats. The space above the diffuser throats 5'7 and between the heat exchange tubes 13 and the hood 15 thus forms a collection chamber for the combustion gases in which their pressure is increased by the dilfuser throats 57 over what it would be with the use of the simple slots 38 in the form of the invention illustrated in FIGURES 13. This increased pressure in the collection chamber increases the flow of combustion gases through the stack or flue 16; or permits modifications in the structure While maintaining the previous flow of combustion gases, such as lessening of the widths of the slots 56 over the Widths of the slots 38, or a reduction in the sizeof the stack leading from the heater, or a reduction in the height of the combustion chamber.

It will be understood that the size of the diffuser throats is not critical and will vary greatly with different size and capacity heaters. By way of example only, it has been found that desirable results are secured when the ratio of the area of the exit of the diffuser throats 57 to the area of the slots 56 is of the order of 3 or 4, and the angle of the diffuser throat for-med by the adjacent flanges 54, 55 may desirably vary between 20 degrees and 30 degrees. Thus, following the examples previously given, the slot 56 may be of the order of A inch wide, the spacing between the upper edges of the baflles 54 and 55 of the order of inch to 1 inch, the angle between the adjacent flanges substantially 20 degrees, and the vertical height of the flanges about 1 inch. These dimensions will be related to heat transfer tubes 13 having an over-all fin diameter of 1% inch.

While the fluid heater according to the present invention has been specifically illustrated and described in connection with the embodiments thereof exemplified in a continuous flow water heater, it will be understood that the invention is applicable to the heating of fluids flowing through heat exchange tubes generally, and whether using products of combustion or other heating fluids; and many variations in the disclosure will be readily apparent to those skilled in the art, and the invention is to be given its broadest interpretation within the terms of the following claims.

I claim:

A l. A fluid heater comprising: a row of substantially parallel heat exchange tubes providing for the passage of a fluid to be heated therethrough; means for passing a heating fluid through said row of heat exchange tubes to effect heat transfer thereto; fins coaxial with said tubes mounted on the exterior surfaces thereof to increase the heat transfer area; bafiles including portions located adjacent to the face of said row of tubes downstream of said heating fluid, said baffle portions extending along the length of the tubes in contact with the edges of the fins on the tubes and being spaced apart to provide slots therebetween substantially aligned with the axes of said heat exchange tubes; and flanges at the adjacent edges of said baffle portions extending downstream of the heating fluid in diverging relation to form diffuser throats for converting the flow energy of the heating fluid passing therethrough into pressure.

2. A fluid heater comprising: a row of substantially parallel heat exchange tubes providing for the passage of a fluid to be heated therethrough; means for passing a heating fluid through said row of heat exchange tubes to effect heat transfer thereto; fins coaxial with said tubes mounted on the exterior surfaces thereof to increase the heat transfer area; and bafiles associated with said row of tubes, said baflles extending along the length of the tubes having substantially flat surfaces thereon facing upstream of said heating fluid and located adjacent to the face of said row of tubes downstream of the heating fluid and disposed parallel to the plane of the row with slots therethrough at the planes through the axes of the tubes extending perpendicular to the row, the edges forming the slots being in substantial contact with the edges of the fins on the tubes to insure passage of heating fluid between the fin surfaces at the downstream face of the row of tubes.

3. A fluid heater comprising: a row of substantially parallel heat exchange tubes providing for the passage of a fluid to be heated therethrough; means for passing a heating fluid through said row of heat exchange tubes to efiect heat transfer thereto; fins coaxial with said tubes mounted on the exterior surfaces thereof to increase the heat transfer area; baffles associated with said row of tubes, said baffles extending along the length of the tubes and having substantially flat surfaces thereon facing upstream of said heating fluid and located adjacent to the face of said row of tubes downstream of the heating fluid and disposed parallel to the plane of the row with slots therethrough at the planes through the axes of the tubes extending perpendicular to the row, the edges forming the slots being in substantial contact with the edges of the fins on the tubes to insure passage of heating fluid between the fin surfaces at the downstream face of the row of tubes; and flanges on said edges extending downstream of said heatiru fluid, the adjacent flanges diverging to form diffuser throats wherein the flow energy of the heating fluid is converted to pressure.

4. A fluid heater comprising: a row of substantially parallel heat exchange tubes providing for the passage of a fluid to be heated therethrough; means for passing a heating fluid through said row of heat exchange tubes to effect heat transfer thereto; fins coaxial with said tubes mounted on the exterior surfaces thereof to increase the heat transfer area; baffles associated with said row of tubes, said baflles extending along'the length of the tubes and having a substantially T-shaped cross section, the cross plates of said baffles being disposed parallel to the plane of the row at the face thereof downstream of the heating fluid with the edges of the cross plates contacting the downstream edges of the fins and with the edges of adjacent bafiles spaced apart in line with the axis of the tubes so as to direct the heating fiuid in contact with the fin surfaces at the downstream face of the row of tubes; and flanges on the edges of said cross plates extending downstream of said heating fluid, the adjacent flanges diverging to form diffuser throats therebetween in which the flow energy of the heating fluid is converted to pressure.

5. A fluid heater comprising: a row of substantially parallel heat exchange tubes providing for the passage of a fluid to be heated therethrough; means for passing a heating fluid through said row of heat exchange tubes to effect heat transfer thereto; fins coaxial with said tubes mounted on the exterior surfaces thereof to increase the the tubes so as to direct the heating fluid in contact with the fin surfaces at the downstream face of the row of tubes, the leg plates of said baffles being directed upstream of the heating fluid and disposed between the fins of adjacent tubes.

6. A fluid heater comprising: a row of substantially parallel heat exchange tubes providing for the passage of a fluid to be heated therethrough; means for passing a heating fluid through said row of heat exchange tubes to effect heat transfer thereto; fins coaxial with said tubes mounted on the exterior surfaces thereof to increase the heat transfer area; and bafiies associated with said row of tubes, said baffles extending along the lengths of the tubes and having a substantially T-shaped cross section, the cross plates of said baffles being disposed parallel to the plane of the row at the face thereof downstream of the heating fluid with the downstream edges thereof contacting the edges of the fins and with the edges of adjacent bafiles spaced apart in line with the axes of the tubes so as to direct the heating fluid in contact with the fin surfaces at the downstream face of the row of tubes, the leg plates of said baffles being directed upstream of the heating fluid and disposed between the fins of adjacent tubes, the spacing between'the edges of adjacent baffles being substantially the minimum distance consistent with effective flow of the heating fluid through the heater.

7. A fluid heater comprising: a substantially horizontal row of heat exchange tubes providing for passage of a fluid to be heated therethrough; a heat source directing a heating fluid upward about said tubes to effect heat transfer to the fluid passing therethrough; balfles above said tubes and extending along the length thereof and spaced apart to provide slots therebetween through which said heating fluid passes; and flanges extending upwardly from the edges of said baffles above said slots, adjacent flanges on opposite sides of the slots diverging upwardly to form diffuser throats therebetween in which the flow energy of the heatin fluid therein is converted to pressure.

8. A fluid heater comprising: a substantially horizontal row of heat exchange tubes providing for passage of a fluid to be heated therethrough; a heat source directing a heating fluid upwardly about said tubes to effect heat transfer to the fluid passing therethrough; and baflies above said tubes and extending along the length thereof, said bafiies having a substantially T-shaped cross section and being disposed with the top plates of the Ts substantially horizontal and above said tubes and with the vertical legs of the Ts disposed between adjacent tubes, the edges of the top plates of adjacent baffles being spaced apart at the vertical planes through the axes of the tubes the minimum distance consistent with effective flow of said heating fluid.

9. A fluid heater comprising: a substantially horizontal row of heat exchange tubes proi/iding for passage of a fluid to be heated therethrough; a fuel burner beneath said tubes to direct products of combustion upwardly through said row to a stack; and baffles above said tubes and extending along the length thereof, said baffles having a substantially T-shaped cross section and being disposed with the top plates of the Ts substantially horizontal and above said tubes and with the vertical legs of the Ts disposed between adjacent tubes, and the tops of adjacent baffles being spaced apart at the vertical plane through the axes of the tubes substantially the minimum distance consistent with proper convention flow of the combustion gases to the stack, the junctions of the top plates and legs of the baflles providing substantially sharp pockets creating turbulence in the combustion gases passing between the tubes.

10. A fluid heater comprising: a substantially horizontal row of heat exchange tubes providing passage for a fluid to be heated therethrough; a fuel burner beneath said tubes to direct products of combustion upwardly through said row to a stack; fins coaxial with said tubes mounted on the exterior surfaces thereof to increase the heat transfer area; and baflies above said tubes and extending along the lengths thereof, said baflles comprising substantially flat plates disposed above said tubes with downward facing flat surfaces having their edges resting on substantially the top edges of the fins, said plates being spaced apart at the vertical planes through the axes of the tubes substantially the minimum distance consistent with efiective flow of combustion gases to the heater stack.

11. A fluid heater comprising: a substantially horizontal row of heat exchange tubes providing passage for a fluid to be heated therethrough; a fuel burner beneath said tubes to direct products of combustion upwardly through said row to a stack; fins coaxial with said tubes mounted on the exterior surfaces thereof to increase the heat transfer area; baflies above said tubes and extending along the length thereof, said baffles comprising substantially flat plates disposed above said tubes with downward facing flat surfaces having their edges resting on substantially the top edges of the fins, said plates being spaced apart at the vertical planes through the axes of the tubes substantially the minimum distance consistent with effective flow of combustion gases to the heater stack; and flanges extending upwardly from the edges of said plates, adjacent flanges diverging upwardly to form diffuser throats therebetween in which the flow energy of the combustion gases passing therethrough is converted to pressure. p

12. A fluid heater comprising: a substantially horizontal row of heat exchange tubes providing for a passage of a fluid to be heated therethrough; a fuel burner beneath said tubes to direct products of combustion upwardly through said row to a stack; fins coaxial with said tubes mounted on the exterior surfaces thereof to increase the heat transfer area; and bafiies above said tubes and extending along the length thereof, said baffles having a substantially T-shaped cross section and being disposed with the top plates of the Ts substantially horizontal and above said tubes and with the vertical legs of the Ts disposed be tween the fins of adjacent tubes, the edges of adjacent baffles being spaced apart at the vertical planes through the axes of the tubes and resting on the edges of the fins at the top of the tubes so as to prevent passage of the products of combustion through the spaces between said bafiie edges without passing between the fin surfaces adjacent to the'tops of the tubes.

13. A fluid heater comprising: a substantially horizontal row of heat exchange tubes providing for passage of a fluid to be heated therethrough; a fuel burner beneath said tubes to direct products of combustion upwardly through said row to a stack; fins coaxial with said tubes mounted on the exterior surfaces thereof to increase the heat transfer area; baflles above said tubes and extending along the length thereof, said baffles having a substantially T-shaped cross section and being disposed with the top plates of the Ts substantially horizontal and above said tubes and with the vertical legs of the Ts disposed between the fins of adjacent tubes, the edges of adjacent baffles being spaced apart at the vertical planes through the axes of the tubes and resting on the edges of the fins at the top of the tubes so as to prevent passage of the products of combustion through the spaces between said baifie edges without passing between the fin surfaces adjacent to the tops of the tubes; flanges extending upwardly from the edges of said bafiies at opposite sides of said vertical planes, adjacent flanges diverging upwardly to form diffuser throats therebetween in which the flow energy of the combustion gases is converted to pressure; and a collection chamber between said heat exchange tubes and the heater stack receiving the combustion gases leaving said diffuser throats.

14. A fluid heater comprising: a substantially horizontal row of heat exchange tubes providing for passage of a fluid to be heated therethrough; a fuel burner beneath said tubes to direct products of combustion upwardly through said row to a stack; fins coaxial with said tubes mounted on the exterior surfaces thereof to increase the heat transfer area; baifles above said tubes and extending along the length thereof, said baffles having a substantially T-shaped cross section and being disposed with the top plates of the Ts substantially horizontal and above said tubes and with the vertical legs of the Ts disposed between the fins of adjacent tubes, the edges of adjacent bafiles being spaced apart at the vertical plane through the axesof the tubes and resting on the edges of the fins at the top of the tubes so as to prevent passage of the products of combustion through the spaces between said bafile edges without passing between the fin surfaces adjacent to the tops of the tubes; and substantially sharp pockets beneath said bafiles at the juncture of the top plates and vertical legs thereof causing turbulence in the flow of combustion gases to prevent stratifying of the heating gases in contact with the fins.

References (Jited in the file of this patent UNIT ED STATES PATENTS OTHER REFERENCES Marks Handbook, Fifth ed. N. Y., McGraw-Hill Book Co., 1951. T] 151 M37. (Page 333 relied on).

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,160,145 December 8 1964 Avy L. Miller It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 6, line 29, strike out "downstream", and insert the same before "edges", first occurrence, in line 30, same column 6; column 7, line 3, for "convention" read convection Signed and sealed this 11th day of May 1965.

(SEAL) Attest:

ERNEST W. SWIDER EDWARD J. BRENNER Attcsting Officer Commissioner of Patents 

8. A FLUID HEATER COMPRISING: A SUBSTANTIALLY HORIZONTAL ROW OF HEAT EXCHANGE TUBES PROVIDING FOR PASSAGE OF A FLUID TO BE HEATED THERETHROUGH; A HEAT SOURCE DIRECTING A HEATING FLUID UPWARDLY ABOUT SAID TUBES TO EFFECT HEAT TRANSFER TO THE FLUID PASSING THERETHROUGH; AND BAFFLES ABOVE SAID TUBES AND EXTENDING ALONG THE LENGTH THEREOF; SAID BAFFLES HAVING A SUBSTANTIALLY T-SHAPED CROSS SECTION AND BEING DISPOSED WITH THE TOP PLATES OF THE T''S SUBSTANTIALLY HORIZONTAL AND ABOVE SAID TUBES AND WITH THE VERTICAL LEGS OF THE T''S DISPOSED BETWEEN ADJACENT TUBES, THE EDGES OF THE TOP PLATES OF ADJACENT BAFFLES BEING SPACED APART AT THE VERTICAL PLANES THROUGH THE AXES OF THE TUBES THE MINIMUM DISTANCE CONSISTENT WITH EFFECTIVE FLOW OF SAID HEATING FLUID. 